1. Field of the Invention
The present invention relates to each of bridge devices in a layer-2 network, a method of controlling the bridge devices, and a control program, and, more particularly, to a bridge device or the like of a layer-2 network that permits the efficient transfer of multicast data.
2. Description of the Related Art
Conventionally, there is a technology that data are distributed by multicasting such as the distribution of moving images to a fixed user terminal connected to a network.
In order to efficiently construct such a network, a multicast routing protocol such as MLD (Multicast Listener Discovery Protocol) that corresponds with IPv6 (Internet Protocol Version 6) and IGMP (Internet Group Management Protocol) that corresponds with IPv4 (Internet Protocol Version 4) is used between the router and host.
In IGMP and MLD, a router sends a Query message at regular intervals and, when a terminal that has received this message wishes to receive multicast data, the terminal sends a Report message to the router. Thereafter, the router confirms the existence of a subordinate multicast reception terminal from the existence of the Report message. The router transfers multicast data of the interface to which the multicast reception terminal is connected only when the multicast reception terminal exists. As a result, an effective application of network resources can be attempted.
Meanwhile, when the network that connects the router and host is a so-called layer-2 network constituted by a plurality of bridge devices, redundant control by a protocol known as STP (Spanning Tree Protocol) is performed within the network.
STP is a protocol standardized by the IEEE (Institute of Electrical and Electronic Engineers) 802.1D. STP is a protocol that uniquely determines a path between bridge devices by suppressing a data frame relay via a predetermined port when a physical redundant path is held between optional bridge devices on the network
More precisely, the path is determined as follows. That is, a control packet known as a BPDU (Bridge Protocol Data Unit) is sent and received between bridge devices and the bridge device having the smallest bridge ID is determined as a root bridge device.
Further, the route is set in a tree shape from the root bridge device to the respective bridge devices and the transfer of data to a path (links) other than this path is broken. As a result, the path between optional bridge devices connected to the network is uniquely determined.
When two or more routes exist between optional bridge devices, the path is determined by selecting the path with the smallest cost (value corresponding with the bandwidth of the network) and breaking the other path.
FIG. 10 shows a constitutional example of a layer-2 network and shows an aspect in which the path is determined by the STP above and multicast data is transferred.
Each of the bridge devices 210 to 250 are mutually connected to a network 100 and the respective bridge devices 210 to 250 are each connected to their respective hosts 110 to 150. A router 170 is connected between the host 110, which is a multicast transmission terminal, and the bridge device 210.
In the case of this example, the bridge 230 is set as a root bridge and a path X between the bridge device 210 and bridge device 250 is broken. Further, because path X is broken when multicast data are transferred from the host 110 to the host 150, data are transferred from the bridge device 210 via the bridge device 220 and so forth.
Conventional technology relating to the transfer of such multicast data includes technology in which two ports connected to either the router or the adjacent switch are set as uplink paths whereby multicast data are always transmitted, and a transmission route between ports is controlled on the basis of a multicast filter table created on the basis of the port receiving a participation message (Japanese Patent Application Laid Open No. 2004-112724, for example).
However, STP determines the root bridge and the break point without conscious of which path the multicast data is communicated, especially conscious of where the router is disposed on the network.
Therefore, when the path shown in FIG. 10 is determined, data is transferred to the bridge device 220 or the like without path X being used. Hence, the usage efficiency of the network 100 becomes very poor. In the case of large-capacity data, such a moving image and so on, the transfer of data to and from another host is sometimes also delayed.
Further, when the path shown in FIG. 10, for example, is also determined by using the multicast filter table created on the basis of participation messages as per Japanese Patent Application Laid Open No. 2004-112724 above, the exact same problems exist.